843 research outputs found
Ferromagnetic ground state of an orbital degenerate electronic model for transition-metal oxides: exact solution and physical mechanism
We present an exact ground state solution of a one-dimensional electronic
model for transition-metal oxides in the strong coupling limit. The model
contains doubly degenerated orbit for itinerant electrons and the Hund coupling
between the itinerant electrons and localized spins. The ground state is proven
to be a full ferromagnet for any density of electrons. Our model provides a
rigorous example for metallic ferromagnetism in narrow band systems. The
physical mechanism for ferromagnetism and its relevance to high-dimensional
systems, like RXMnO, are discussed. Due to the orbital
degeneracy of itinerant electrons, the superexchange coupling can be
ferromagnetic rather than antiferromagnetic in the one-band case.Comment: 4 page, no figure To appear in Phys. Rev. B, (January 1, 1999
An effective Hamiltonian for an extended Kondo lattice model and a possible origin of charge ordering in half-doped manganites
An effective Hamiltonian is derived in the case of the strong Hund coupling
and on-site Coulomb interaction by means of a projective perturbation approach.
A physical mechanism for charge ordering in half-doped manganites
(R_{0.5}X_{0.5}MnO_3) is proposed. The virtual process of electron hopping
results in antiferromagnetic superexchange and a repulsive interaction, which
may drive electrons to form a Wigner lattice. The phase diagram of the ground
state of the model is presented at half doping. In the case of formation of
Wigner lattice, we prove that spins of electrons are aligned ferromagnetically
as well as that the localized spin background is antiferromagnetic. The
influence of the on-site Coulomb interaction is also discussed.Comment: 6 pages ReTex with two figures To appear in Phys. Rev. B 59, (June 1,
1999
The Glashow resonance as a discriminator of UHE cosmic neutrinos originating from p-gamma and p-p collisions
We re-examine the interesting possibility of utilizing the Glashow resonance
(GR) channel nu_ebar + e^- to W^- to anything to discriminate between the UHE
cosmic neutrinos originating from p-gamma and pp collisions in an optically
thin source of cosmic rays. We propose a general parametrization of the initial
neutrino flavor composition by allowing the ratios Phi^{p gamma}_{pi^-}/Phi^{p
gamma}_{pi^+} and Phi^{pp}_{pi^-}/Phi^{pp}_{pi^+} to slightly deviate from
their conventional values. A relationship between the typical source parameter
kappa = (Phi^{p gamma}_{pi^+} + Phi^{p gamma}_{pi^-})/(Phi^{pp}_{pi^+} +
Phi^{pp}_{pi^-} + Phi^{p gamma}_{pi^+} + Phi^{p gamma}_{pi^-}) and the working
observable of the GR R_0 = Phi^T_{nu_ebar}/ (Phi^T_{nu_mu} + Phi^T_{nu_mu}) at
a neutrino telescope is derived, and the numerical dependence of R_0 on kappa
is illustrated by taking account of the latest experimental data on three
neutrino mixing angles. It is shown that a measurement of R_0 is in principle
possible to identify the pure p-gamma interaction (kappa =1), the pure pp
interaction (kappa =0) or a mixture of both of them (0 < kappa < 1) at a given
source of UHE cosmic neutrinos. The event rate of the GR signal against the
background is also estimated.Comment: 13 pages, 6 figures, final version to appear in Phys. Rev.
Updated Values of Running Quark and Lepton Masses
Reliable values of quark and lepton masses are important for model building
at a fundamental energy scale, such as the Fermi scale M_Z \approx 91.2 GeV and
the would-be GUT scale \Lambda_GUT \sim 2 \times 10^16 GeV. Using the latest
data given by the Particle Data Group, we update the running quark and
charged-lepton masses at a number of interesting energy scales below and above
M_Z. In particular, we take into account the possible new physics scale (\mu
\sim 1 TeV) to be explored by the LHC and the typical seesaw scales (\mu \sim
10^9 GeV and \mu \sim 10^12 GeV) which might be relevant to the generation of
neutrino masses. For illustration, the running masses of three light Majorana
neutrinos are also calculated. Our up-to-date table of running fermion masses
are expected to be very useful for the study of flavor dynamics at various
energy scales.Comment: 23 pages, 6 tables, 2 figures; version published in PR
Neutrino Masses, Lepton Flavor Mixing and Leptogenesis in the Minimal Seesaw Model
We present a review of neutrino phenomenology in the minimal seesaw model
(MSM), an economical and intriguing extension of the Standard Model with only
two heavy right-handed Majorana neutrinos. Given current neutrino oscillation
data, the MSM can predict the neutrino mass spectrum and constrain the
effective masses of the tritium beta decay and the neutrinoless double-beta
decay. We outline five distinct schemes to parameterize the neutrino
Yukawa-coupling matrix of the MSM. The lepton flavor mixing and baryogenesis
via leptogenesis are investigated in some detail by taking account of possible
texture zeros of the Dirac neutrino mass matrix. We derive an upper bound on
the CP-violating asymmetry in the decay of the lighter right-handed Majorana
neutrino. The effects of the renormalization-group evolution on the neutrino
mixing parameters are analyzed, and the correlation between the CP-violating
phenomena at low and high energies is highlighted. We show that the observed
matter-antimatter asymmetry of the Universe can naturally be interpreted
through the resonant leptogenesis mechanism at the TeV scale. The
lepton-flavor-violating rare decays, such as , are also
discussed in the supersymmetric extension of the MSM.Comment: 50 pages, 22 EPS figures, macro file ws-ijmpe.cls included, accepted
for publication in Int. J. Mod. Phys.
Thorium-doping induced superconductivity up to 56 K in Gd1-xThxFeAsO
Following the discovery of superconductivity in an iron-based arsenide
LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc
was pushed up surprisingly to above 40 K by either applying pressure[2] or
replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to
55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was
found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands
for the lanthanide elements) at an apparently optimal doping level. However,
the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice
mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation
of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+
in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice
mismatch, hence induces the high temperature superconductivity.Comment: 4 pages, 3 figure
Bayesian Computing with INLA: A Review
The key operation in Bayesian inference is to compute high-dimensional integrals. An old approximate technique is the Laplace method or approximation, which dates back to Pierre-Simon Laplace (1774). This simple idea approximates the integrand with a second-order Taylor expansion around the mode and computes the integral analytically. By developing a nested version of this classical idea, combined with modern numerical techniques for sparse matrices, we obtain the approach of integrated nested Laplace approximations (INLA) to do approximate Bayesian inference for latent Gaussian models (LGMs). LGMs represent an important model abstraction for Bayesian inference and include a large proportion of the statistical models used today. In this review, we discuss the reasons for the success of the INLA approach, the R-INLA package, why it is so accurate, why the approximations are very quick to compute, and why LGMs make such a useful concept for Bayesian computing
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